1. Field of the Invention
The present invention relates generally to ceramic circuit structures such as thin tapes and sheets. More particularly, the present invention is related to providing ceramic green sheets of controlled microporosity.
2. Description of the Prior Art
In general, ceramic green sheets are prepared from ceramic paints by mixing a ceramic particulate, a thermoplastic polymer and solvents. The paint is then cast or spread into ceramic sheets or slips from which the solvents are subsequently volatilized to provide a coherent and self-supporting flexible ceramic green sheet, which may be finally fired to drive off the resin and sinter the ceramic particulates into a densified ceramic substrate.
In the fabrication of multilevel structures, an electrical conductor forming composition is deposited in a pattern on ceramic green sheets which form components in the multilevel structure. The component green sheets may have via or feed-through holes punched in them. The required number of component green sheets are stacked or superimposed in register on each other in the desired order. The stack of green sheets is then compressed or compacted at a necessary temperature to effect a bond between adjacent layers of the green sheets in the portions between adjacent layers not separated by the electrical conductor forming pattern. Thereafter, the green sheet laminate is fired to drive off the binders and to sinter the ceramic dielectric structure having the desired pattern of electrical conductors extending internally therein.
It is generally considered essential (as elaborated more fully in U.S. Pat. Nos. 2,966,719 and 3,125,618) that the density of the fired ceramic approach the theoretical possible figure for the raw material (e.g. ceramic particulate) selected, and also that the ceramic product be non-porous without formation of micropores in order to prevent detrimental effect upon the electrical characteristics thereof. The formation of such non-porous fired ceramics results in a void-free densification of the ceramic particulate in green sheets which when sintered provides a densified ceramic product. However, it should be noted that although such densified ceramic green sheets have been found satisfactory for single level ceramic structures, they nevertheless provide serious problems in attempts to laminate them into multilevel structures, particularly where electrical conductor patterns are sandwiched or otherwise incorporated between levels.
As will be evident, a pattern of electrical conductors when coated on a green sheet defines a relief pattern whose top surface is raised relative to the uncoated surface of the green sheet. Thus, in laminating a second superimposed green sheet on a conductor patterned green sheet, it will be necessary to compress the two green sheets together to bring uncoated adjacent portions of the green sheets in contact with one another so that the portions may be bonded to form the desired integrated or unified ceramic laminate or structure (See Nufer 1992, "Ceramic Greensheet Technology For Glass-Ceramic/Copper Multilevel Substrates, Electronic Components and Technology Conference, San Diego, Calif., 42:673-677).
Although the binder resin characterizes the green ceramic sheet with some degree of pliancy and ductility, any extended flow or extrusion of individual green sheets in the stack within their plane under compression, will necessarily be attended by distortion elongation and/or possible rupture of any electrical conductor pattern which may be contained between adjacent green sheets in the stack. Accordingly, it is essential that the green sheets employed in the fabrication of a multilayer ceramic must be characterized by dimensional stability within their plane which precludes lateral flow of the green ceramic, if the integrity of the conductor pattern is to be maintained, and to insure registration of the green ceramic laminae of the stack. As a consequence, it is necessary that any distortions of a stack of green sheets under compression be substantially limited in the vertical planes when the uncoated sections of adjacent green sheets are brought into contact for bonding while closely conforming about the conductor pattern to insure complete conductor line enclosure.
Green sheet compositions available heretofore have not been amenable to compressive bonding to each other due to the inherent resiliency of the binder systems employed for the ceramic particulate. As a consequence, upon release of compression, the resiliency of the binder system is characterized with an elastic rebound or spring-back frequently accompanied by rupture of the bonded interface between adjacent green sheet laminae in the stack.
Accordingly, it is considered essential that a green ceramic sheet be provided for multilayer structures having lateral dimensional stability with sufficient compressibility to enable a necessary set to permit bonding to each other about an enclosed raised conductor pattern, while maintaining the desired degree of densification consonant with necessary electrical and dielectric characteristics.
As mentioned above, although densified ceramic green sheets are found satisfactory for single level ceramic structures, they pose serious problems when laminated into multilevel structures, particularly where electrical conducting patterns are sandwiched or otherwise incorporated between levels. Hence, controlled microporosity is essential for constructing reliable and stable multilevel structures.